Late blight, caused by the oomycete Phytophthora infestans, is one of the most serious diseases in worldwide potato production. It was responsible for the Irish potato famine of the mid-19th century, resulting in the death of one million people. Although a lot of effort has been invested in controlling the pathogen, chemical control of P. infestans is still the main crop management strategy, but environmental safety is becoming more important and the pathogen is sometimes able to evolve resistance to the fungicide treatment. Therefore, introduction of resistance into modern potato varieties is the most durable strategy to control the disease.
In the last century, Solanum demissum, which is a hexaploid Mexican species, was extensively used in breeding for late-blight resistance in potato. Initially, a series of 11 R genes derived from S. demissum was described. Of these, R1, R2, R3a/b, R6, and R7 have been localized on the genetic maps of potato (Solanum tuberosum). However, these R genes confer pathovar-specific resistance and those that were introgressed into potato varieties, mainly R1, R2, R3, R4, and R10, were quickly overcome by the pathogen. Hence, new sources for resistance are required, and currently, several other wild Solanum species have been reported as being potential sources of resistance, many of which have been genetically characterized (Table 8).
Recent efforts to identify late blight resistance have focused on major R genes conferring broad-spectrum resistance derived from diverse wild Solanum species. Beside S. demissum, other wild Solanum species such as S. acaule, S. chacoense, S. berthaultii, S. brevidens, S. bulbocastanum, S. microdontum, S. sparsipilum, S. spegazzinii, S., stoloniferum, S. sucrense, S. toralapanum, S. vernei and S. verrucosum have been reported as new sources for resistance to late blight (reviewed by (Jansky, 2000)).
S. x edinense P. Berthault, a pentaploid (2n=5x=60) potato species from Mexico, is a natural hybrid between the Mexican Solanum demissum and the South American S. tuberosum spp. andigena. The pentaploid S. x edinense had been identified as an interesting source of resistance to P. infestans already in 1908 by Salaman and was included in breeding programs by Brioli in 1914 (Pavek et al. 2001; Toxopeus 1964). It was named after the Edinburgh Botanic Garden (Glendinning 1983), where its hybrid characteristic was first described. It has been used in breeding programs and has revealed good field resistance to P. infestans (Van Soest et al. 1984). Two functional R genes have been cloned from one S. x edinense genotype (edn151-3): Rpi-edn1.1 and Rpi-edn1.2 also known as R2-like (Champouret 2010). They were identified by allele mining of the R2 family. Both are located in the R2 cluster on chromosome 4. Both R genes recognize AVR2 (Champouret 2010; Lokossou et al. 2009) and their resistance is not effective against all P. infestans isolates, including IPO-C (Lokossou et al. 2009).
To date, not only from this species, but also from other Solanum species late blight R-genes have been cloned, like the allelic genes RB and Rpi-blb1 on chromosome 8 and Rpi-blb2 on chromosome 6 (Table 6) of S. bulbocastanum. Recently, also an Rpi-blb3 resistance gene has been isolated (WO 2008/091153). Also a resistance gene of S. chacoense has been characterized (EP 09170769.5). Although the initial results obtained with RB and Rpi-blb1, -2 and -3 are promising, there is a further need for additional R-genes, especially because allele mining of these genes in S. bulbocastanum genotypes revealed that natural stacking of Rpi-blb1, and -3 in a single genotype occurs at relatively high frequency (Lokossou 2010). S. venturii is another example of the presence of several R genes with different specificities in a single genotype (Pel 2010). Stacking several R genes in a single genotype appears to be a feasible strategy to achieve high level and durable protection against potential pathogens. Pyramiding of R genes is still controversial and it is not known whether it is a durable approach (McDowell et al. 2003; Pink et al. 1999; Pink 2002). The pyramiding of Rpi-ber1 (Rauscher et al. 2006), an R gene with a strong effect, and Rpi-mcd1 (Tan et al. 2008), an R gene with a weak effect, revealed an additive effect on the resistance level (Tan et al. 2010). Observing natural pyramiding of R genes strengthens the idea that plants can benefit from combining individual R genes, even including some with weaker effect (Pink 2002).